1. Field of the Invention
The present disclosure generally relates to geothermal power plants. More particularly, the disclosure relates to corrosion inhibitors used in geothermal systems.
2. Description of the Related Art
Geothermal energy is energy in the form of heat within the earth's interior, which is tapped by geothermal wells. Since the earth's interior is extremely hot, there is an enormous potential energy supply. However, there are many technical and economic challenges in optimizing the tapping of this energy source. The use of geothermal energy as a renewable energy source, nonetheless, has gained in importance as other energy sources become less abundant and more expensive.
Geothermal energy moves towards the earth's surface by thermal conduction through solid rock. Thermal energy can also be transmitted towards the earth's surface by movement of molten rock or by circulation of fluid (H2O as steam or water) through interconnected fractures and pores, which may provide heat reservoirs closer to the surface, and thus a site more accessible to drilling for wells to tap geothermal energy.
Natural geothermal reservoirs, on which many commercial geothermal wells are located, comprise volumes of rock at high temperatures (up to about 350° C. or 622° F.) and often also of high porosity and high permeability to fluids. Wells are drilled into such a reservoir and the thermal energy in the rock is transferred by conduction to a fluid (H2O as water or steam), which subsequently flows to the well and then up to the earth's surface. In areas where the rock has a low porosity and permeability, it must be artificially fractured by means of explosives or hydrofracturing to provide a network of such fractures, commonly known as Enhanced Geothermal Systems (EGS).
The thermal fluid within the fractures and pores of a reservoir may be almost entirely in a liquid state, which liquid state exists at temperatures much higher than the boiling point of water at atmospheric pressure because of the high pressure of overlying water. Such a reservoir is referred to as a liquid-dominated, or water-dominated, reservoir. When the thermal fluid within larger fractures and pores is in the form of steam, the reservoir is referred to as a vapor-dominated reservoir. A liquid-dominated reservoir may produce either water or a mixture of water and steam. A vapor-dominated reservoir routinely produces only steam, and in most instances the produced steam is super-heated steam.
In the geothermal production of electricity from a water-dominated reservoir, the pressurized hot water or wet steam produced from a well is flashed to a lower pressure at the earth's surface, separating steam or converting the water partly to steam, and this steam is used to drive a conventional turbine-generator set. In a relatively rare vapor-dominated reservoir, the superheated steam may be piped directly to the turbine without the separation of water.
Many geothermal wells for the production of electricity are water-dominated hydrothermal convection systems characterized by the circulation of surface water, including wastewaters and/or condensates, downhole. The driving force of the convection systems is gravity, the cold downward-moving recharge water being much denser than the heated, upward-moving thermal water. The technique of reinjection of wastewaters or condensates back into the wells may be used for a number of reasons, including avoidance of surface disposal of such waters which may contain pollutants. Selective injection or reinjection of water into the thermal system may help to retain aquifer pressures and to extract more geothermal energy from the rock than is possible when fresh geothermal water is itself the main produced fluid. The produced fluid is either magmatic (released from solidifying magma), meteoric (rain and snow), or a mixture of the two, and may be fresh, reinjected, or a mixture of the two.
Geothermal steam is generally used as the energy source, regardless of whether the produced fluid is steam, partly steam, or water that is partly converted to steam flash evaporation. Geothermal steam is used in power generation as well as heating and electrical processes. Geothermal steam temperatures range from about 185° C. to about 370° C. (about 365° F. to about 700° F.), have a salinity from less than 1000 ppm up to several hundred thousand ppm, and a content of non-condensable gases (NCG) up to about 6 percent. Much higher temperature fluids can be extracted from the ground using deeper wells.
While geothermal power plants remain attractive from an environmental perspective, corrosion control in geothermal power production is a major hurdle in advancing the use of this renewable source of energy. Among the many challenges is the variability of brine chemistry, the use of multiple wells, the temperature, NCG, and the materials used in the process. In hot-brine based geothermal plants, pH modification processes are used to prevent silica scale formation on the surface equipment and injection wells. The scale prevention methods for the pH modification process include reducing the pH of the brine by adding mineral acid, such as hydrochloric acid (HCl) to achieve a pH of about 4.5 to about 5. The mineral acid may be added at a location between the high pressure separator and the low pressure separator. In the low pressure separator, the pH may be further lowered to about 2.5.
The low pH environment is conducive to corrosion, in addition to the high temperatures, high total dissolved solids (TDS), and dissolved gasses such as H2S and CO2.